1. Field of the Invention
The invention relates to an apparatus and a method for detecting an abnormality of imbalance of air-fuel ratios among cylinders of a multi-cylinder internal combustion engine, and more particularly, to an apparatus and a method for detecting that there is a relatively great imbalance of air-fuel ratios among cylinders in a multi-cylinder internal combustion engine.
2. Description of the Related Art
In an internal combustion engine equipped with an exhaust gas purification system that utilizes a catalyst, noxious components in exhaust gas are purified by the catalyst with high efficiency. It is therefore indispensable to control the mixing ratio of air and fuel in the mixture burned in the internal combustion engine, namely, the air-fuel ratio. In order to perform this air-fuel ratio control, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine. Feedback control is performed such that the detected air-fuel ratio coincides with a predetermined target air-fuel ratio.
In a multi-cylinder internal combustion engine, when air-fuel ratio control is performed using the same controlled variable for all cylinders, there is caused an imbalance of actual air-fuel ratios among the cylinders in spite of the performance of air-fuel ratio control. If the degree of this imbalance is small, the imbalance can be absorbed through air-fuel ratio feedback control, and noxious components in exhaust gas can be subjected to a purification treatment in the catalyst as well. Therefore, the exhaust emission properties are not influenced.
However, if there is caused a great imbalance of air-fuel ratios among the cylinders due to, for example, a malfunction in a fuel injection system of one or some of the cylinders or the like, the exhaust emission properties may be deteriorated. Such a great imbalance of air-fuel ratios as to deteriorate the exhaust emission properties is desired to be detected as an abnormality. Especially in the case of an internal combustion engine for a vehicle, with a view to preventing the vehicle from traveling with the exhaust emission properties deteriorated, there have been demands to detect an abnormality of imbalance of air-fuel ratios among the cylinders onboard.
For example, according to the disclosure of Japanese Patent Application Publication No. 2010-112244 (JP-2010-112244 A), it is determined, on the basis of a calculated value of air-fuel ratio feedback control, whether or not there is an air-fuel ratio abnormality. If it is determined that there is an air-fuel ratio abnormality, the time of fuel injection into each of cylinders is reduced by a predetermined time, and only that one of the cylinders whose air-fuel ratio is abnormal is misfired.
For example, it is conceivable to detect a parameter regarding rotational fluctuations of each of the cylinders, and detect whether or not there is an abnormality of imbalance of air-fuel ratios among the cylinders on the basis of this detected parameter.
In recent years, internal combustion engines having cylinders in each of which the working angle (the crank angle from a valve open state to a valve closed state) of an intake valve is variable have been put into practical use.
In this type of internal combustion engine, if an attempt is made to detect an abnormality of imbalance on the basis of a parameter regarding rotational fluctuations, the following phenomenon may be caused. That is, the cause of an abnormality of imbalance of air-fuel ratios among the cylinders includes an injection amount deviation as a deviation of the amount of fuel injection from a normal value in a specific one of the cylinders, and an air amount deviation as a deviation of the amount of intake air from a normal value in a specific one of the cylinders. In the case of an injection amount deviation, especially in the case where the amount of fuel injection is deviant from the normal value toward a decreasing side, there is no influence of the working angle of an intake valve. Therefore, it is possible to determine whether the air-fuel ratios are normal or abnormal regardless of the working angle.
In the case of an air amount deviation, especially in the case where the amount of air is excessively small due to a deviation of the working angle from a normal value toward a decreasing side, the result of the determination may differ depending on the magnitude of the working angle as a non-deviant normal value. That is, in the case where the working angle as a normal value is small, there is a major influence when the working angle is deviant toward the decreasing side. Therefore, it is possible to accurately determine whether the air-fuel ratios are normal or abnormal.
However, in the case where the working angle as a normal value is large, there is a minor influence when the working angle is deviant toward the decreasing side. Thus, it may be erroneously determined that the air-fuel ratios are normal, even if the air-fuel ratios are abnormal. For this reason, it may become difficult to ensure a sufficient detection accuracy.